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I've been given some notes and I have to 'unscramble' them and put them in order. They are supposed to describe what happens in the diagram below:

enter image description here

The notes to unscramble and form a proper answer from are below:

  • Field lines
  • Once it is pushed until 90 degrees then passes it
  • It turns clockwise
  • Then reverses motion and oscillates
  • Comes to a rest vertically

Now, I understand that the order of things happening should be as follows, using the notes above, with little explanations to make a complete answer:

  1. The magnet's poles cause magnetic field lines to go from north to south
  2. Using the left-hand rule for each side of the coil, we can see that the forces do indeed go in the directions shown in the diagram. (Denoted in the diagram with "F" and arrows).
  3. This causes the coil to rotate clockwise.
  4. When it has rotated 90 degrees clockwise, it comes to a rest vertically.
  5. It then reverses direction and is pushed past 90 degrees backwards, anticlockwise. And so it will continue to oscillate, reversing direction every time it gets to the vertical position.

Am I correct?

If so, why exactly does it reverse direction? Please if someone could clarify this explicitly. I've been to many videos and tutorials that explain similar things, but mostly in the context of proper DC motors (and this is not a DC motor), but they keep confusing me.

On the other hand, if the coil doesn't reverse direction at all but continues in the same direction, please let me know!

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2 Answers 2

It doesn't reverse the direction. Inertia keeps the coil moving past the vertical position and as soon as it is past the vertical position, the forces come into play pushing it in the same sense as before.

Edit: Unless (which I doubt), the question is considering the torsional forces acting on the loop. It's possible that the loop turns with the battery part remaining stationary. This would result in a twist, which would push it back the opposite direction. However, I don't think the question includes this.

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In a DC motor, the current coil doesn't reverse, but isn't that only because a motor has a split-ring commutator at the end of the coil? And it's because of the commutator that the current reverses, and so the coil keeps going on in the same direction despite the change in direction of the current. But in this case there is no commutator, so the coil shouldn't reverse direction, right? –  user961627 May 18 at 13:36
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@user961627 OH, I see. They assume the whole circuit to be turning along with the coil. I assumed the presence of a commutator ring, or that only the loop rotates. Yes, with the entire loop rotating, after half a turn, the forces would start acting in the opposite direction because the battery is now - effectively - reversed. –  mikhailcazi May 18 at 14:07
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Okay I think I got my answer! But please correct me if wrong.

The main issue is that the diagram above is very much like a DC motor, but is missing a commutator. This page explains it in relation to a DC motor: http://nsb.wikidot.com/pl-9-3-1-6

The reason why a DC motor doesn't reverse direction is because of the split ring commutator. The diagram above doesn't include a commutator, so the coil can't help but reverse direction.

About the commutator, from the link above:

  • This is a device that reverses the direction of a current flowing through an electric motor.
  • The role of this commutator is to keep the current in the armatures relatively constant to the magnetic field, ensuring that it will spin in the same direction as long as the motor is functioning.
  • Without the Split Ring Commutator, the current would cause the rotors to spin in the opposite direction after reaching the zero torque point (RH Rule) and it will oscillate until it stops perpendicular to the field.
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Yes, this is the correct answer. :) –  mikhailcazi May 18 at 14:08

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